A cogeneration system using a fuel cell or an engine can recover heat generated at the same time as electric power generation as hot water to effectively utilize an energy and is attracting attention as a highly efficient distributed power supply. For example, as shown in FIG. 12, in the case of a fuel cell cogeneration system, normally, heat generated by a fuel cell 21 is recovered by cooling water in a cooling water passage 22, and the cooling water exchanges the heat with hot water in a hot water passage 23 via a heat exchanger 24. The hot water heated in the heat exchanger 24 is stored in the hot water tank 27 as heated water, and the heated water is supplied from an exit of the hot water tank 27 through a hot water supplying passage 28 to a heat load in accordance with a heat demand of the heat load. Proposed is that in this fuel cell cogeneration system, a surplus electric power heater 25 configured to heat the hot water using surplus electric power which is a part of electric power generated by the fuel cell 21 and is unconsumed by an electric power load is disposed on a hot water passage 25 located downstream of the heat exchanger 24, and an auxiliary heat source 29 is disposed on the hot water supplying passage 28 as a backup in a case where the amount of heated water in the hot water tank 27 is not enough with respect to the heat demand of the heat load (see Patent Document 1 for example).
Patent Document 1: Japanese Laid-Open Patent Application Publication 2005-12906